As fastener joints in work pieces have become more highly stressed, especially in the aerospace industry, a great deal of effort has been expended in an attempt to increase the fatigue life of the fastener joints. One of the most successful techniques that has been used is commonly referred to as coldworking in which the holes through the work pieces are expanded beyond the elastic limit of the material of the work pieces prior to the installation of the fastener through the holes. This causes a compressive stress gradient to be induced in the work pieces immediately adjacent the holes that serves to increase the fatigue life of the resulting joint. Several techniques have been conceived which coldwork the holes such as those disclosed by U.S. Pat. Nos. 3,270,410; 3,434,327; 3,566,662; 3,805,578; and 3,835,688. All of these techniques use a signel piece mandrel which is pulled or pushed through the holes either with or without a sleeve member.
Because these mandrels have been made of one piece, however, it has been difficult to achieve the necessary hardness in the enlarged portion of the mandrel which contacts and expands the hole while at the same time being able to achieve the necessary toughness in that portion of the mandrel which is used to pull the mandrel through the hole. As a result, the prior art has had to resort to the use of exotic lubricants and to the use of sleeve members to isolate the surface of the hole from the expansion surface of the mandrel in order to be able to physically pull the mandrel through the hole and also not damage the hole.
Also the expansion surface has frequently become damaged requiring the entire mandrel to have to be resurfaced to a smaller expansion diameter or thrown away. There was also no way to insure that an oversize mandrel would not be inadvertently pulled through the holes which may render the work piece unusable.
When threaded fasteners are installed in the holes through work pieces, the torque applied to the nut as it is screwed onto the fastener to lock the fastener in place has been used to apply the desired preload to the fastener joint. Because the friction between the nut threads and the fastener threads, between the base of the nut and work piece or washer under the nut, and between the bearing surface of the fastener and hole surface cannot be accurately determined and may vary from joint to joint; the amount of preload actually applied to the fastener joint has varied widely even though the same amount of torque was used to tighten each nut.